Uranium prospecting may be done by drilling a well similar to that drilled in oil exploration. Modern technology has led to many methods for obtaining radioactive material from the well once it is located.
A common technique for locating the level at which radioactive material is present is to "log" the well or measure neutrons emitted by the formations surrounding the wellbore. This is accomplished by lowering a logging tool such as a neutron detector down to the bottom of the well. The tool may then be pulled up the well while measurements are taken of the radioactivity which gives an indication of the lithology. In certain applications, a neutron source is included in the logging tool to inject neutrons into the formations. The injected neutrons will cause any radioactive material present to emit secondary neutrons. When secondary neutrons are detected, radioactive material is located. Unfortunately, the burst of neutrons injected into the formation causes the generation of gamma-rays greatly in excess of the secondary neutrons.
Traditionally, a neutron detector is used such as that illustrated in FIG. 1 to obtain a count of neutrons present. Neutron detector 12 generally consists of a metal cylinder 14 which contains a detector gas 16, typically 3He. A metal rod 18 is located in the center of cylinder 14 and connected to the anode of a DC power supply 20 through resistor 21. The cathode of power supply 20 is connected to metal cylinder 14.
In operation, neutrons 22 penetrate metal cylinder 14 and collide with 3He gas molecules within. An electron 24 is readily given up by the force of the collision. The freed electrons 24 are attracted to the positively charged rod 18. The presence of electrons is measured as an increased current flow measured at node 26 through capacitor 28. Each electron causes a change in amplitude of the current measured.
Unfortunately, the amplitude change is not exactly the same for each electron and it has been discovered that high speed gamma-rays can free electrons 24 from cylinder 14. Electrons freed by high speed gamma-rays may occasionally cause an amplitude change approximately equal to that caused by neutron collisions. With the highly sensitive neutron detections of modern technology, an inaccurate neutron count will result.